Lidar systems, as well as other time-of-flight ranging sensors, face a trade off between measurement range and measurement rate due to multiple time around (MTA) ambiguity. MTA occurs when the lidar device takes measurements at a rate fast enough that previous pulses of energy may return from distant targets after successive pulses have been transmitted. This causes multiple pulses to be in the air at once. When the lidar system receiver measures a return, it does not know to which pulse the return corresponds so it cannot calculate the correct time of flight for the measurement. This means that lidar systems are constrained in how they can measure the world. They can either measure far off distances or sample the world densely. Airborne vehicles moving quickly need both as much as possible in order to perceive the world and plan safe trajectories. This problem particularly affects long-range forward and sideward scanning sensors for aircraft that scan in pitch, as the distance of targets varies greatly between terrain below the aircraft and terrain on the horizon. The potential measurement rate for the close targets suffers because the lidar system must pulse slowly to avoid MTA when pointed towards distant targets on the horizon.
One solution is to use multiple lidar systems in an aircraft, but this may not be acceptable in some aircraft applications because of size, weight, power and/or cost system budgets.